Installation for transmitting alarm signals

ABSTRACT

In an installation for transmitting alarms, preferably in connection with attacks on persons, and for locating the alarm sender, there is at least one portable alarm sender 2 sending an alarm to a central alarm receiver (3) by radio. The senders each contain a memory (24) for a code unique to their position in the installation. The code is automatically set by fixed transmitters (1) having a small range, which transmit it electromagnetically, especially inductively, to the sender (2), the code being unique to the location of a transmitter and stored in the sender memory. When fixed transmitters (1) are arranged so close together that their unique codes are difficult to separate in the sender (2), a dummy transmitter (56) is arranged to prevent alteration of a code already stored in the memory (24) of the alarm sender, as long as the sender (2) is close to the dummy transmitter (56). The sender (2) also transmit a second code stored in a second memory, unique to the individual alarm sender and thereby to the carrier of the alarm sender.

FIELD OF THE INVENTION

The present invention relates to an installation for transmitting alarmsignals, primarily in cases of personal attack, and localizing alarmtransmitters.

DESCRIPTION OF PRIOR ART

For the staff of hospitals, prisons and other custodial institutionsthere is a need for calling for help in assault or battery by means ofan alarm sender which can be carried in a pocket.

In such installations it is important that the place from which an alarmhas been sent can be localized. Localization can be done with aid of analarm receiver, so that every room in a building is provided with analarm receiver. In such case since only the alarm receiver in the roomwhere the alarm signal has been sent reacts to an alarm, localization iscarried out simply. Thus the propagation of the alarm signal is limitedby the walls of the room when the alarm signal is sent supersonically orby infrared light. A serious drawback with alarm transmission in thismode is that transmission is hindered or heavily dampened if the alarmsender is kept under clothes or if it were to be under the body during astruggle. It should specifically not be necessary to take out the alarmsender in an assault, since this itself can initiate the aggressiveaction.

If a radio transmitter is used instead for sending the alarm, it can becarried in the clothes and does not need to be taken out to send analarm, since radio transmission penetrates through clothes quite easily.The good penetration ability of radio transmission, however, excludeslocalization of the alarm sender with the help of receivers in eachroom, since the emissions from the transmitters are not limited by thewalls of a room. Attempts to localize such an emission by takingbearings is made impossible by all the reflections obtained from thebuilding walls.

It would be easy to localize the radio transmitter sending an alarm ifthe transmission contained a code notifying the location of the alarmsender, but requiring staff to set a variable room code on their alarmsenders is not reasonable in practice. Staff should not need to thinkabout the alarm sender except at the moment when it needs to be used.

SUMMARY OF THE INVENTION

According to the present invention, a room code is automatically set inthe alarm sender, giving its position in the building. The set room codeis automatically changed when the alarm sender is carried from one placeto another. When an alarm is sent, the latest stored room code is sentby radio to a central alarm receiver. The room code received in thealarm receiver is stored in a memory and the position of the sender isshown on a digital display. How coded messages are sent by radio andreceived and displayed in a central receiving station is alreadydescribed in the U.S. Pat. No. 3,678,391.

The room code in the alarm senders is reset as follows: A number ofsmall, locally fixed transmitters, with continuous electromagneticemission, in this case inductive emission, are placed in a building, andsend a code unique for the position of the fixed transmitter to areceiver in each alarm sender, this code therefore being called the roomcode. The received code is stored in a memory in the sender, and if thesender is triggered for sending an alarm, the stored code is transmittedwith the radio signal to the central alarm receiver.

Each fixed transmitter is suitably placed in one door frame so thattransmission of the room code takes place when the alarm sender passesthrough door frames between different rooms. The code transmitted by thefixed transmitter is thus unique for the door frame where thetransmitter is placed. The fixed inductive transmitters for the uniquecode only transmit with low power, and their transmitting antenna is asmall magnetic dipole antenna. Since the transmitting power is low andthe field strength from a small dipole antenna is, to the firstapproximation, inversely proportional to the cube of the distance fromthe antenna, the clear domination of the field strength from atransmitter in a door frame over the field strength from transmitters inother door frames is thus ensured.

For wide doors it may be necessary for a transmitter to feed two dipoleantennae, one in each door jamb.

Inductive transmission of the room code has been selected not onlybecause it penetrates clothes without difficulty, so that the alarmsender can be carried in a pocket, but also because the propagation ofthe magnetic field from the dipole antenna of the fixed transmitter canbe accurately calculated and limited to the areas nearest to a dooropening.

The inductive receiver for receiving the room code in a portable alarmsender will be alternately exposed to strong fields at the door openingsand almost no fields at a short distance therefrom, and sometimes to theinteraction of nearly equal strong fields from two inductivetransmitters. This results in that the inductive receiver of the alarmsender alternatingly receives strong signals having correct codes andsignals with incorrect, interrupted or mixed codes. Parity bits aretherefore added to the room codes, and a decoder placed in the inductivereceivers of the alarm sender are adapted for reading the parity bitsand discovering errors in the received codes. Incorrect codes will thusbe rejected and not allowed to alter a room code already stored in thememory of the alarm sender.

If a doorway is placed very close to the wall of a room, other than theone to which the doorway in question leads, the field strength from thetransmitter in the doorway can be stronger in a portion of the firstroom than the field strength from the transmitter in the door frame ofthis room. To prevent unwarrented resetting of the alarm sender code atany place in the first room, a dummy sender can be arranged, which isadapted to transmit a signal preventing alteration of a unique codealready stored in the memory of the radio transmitter.

Receivers for inductively transmitted coded signals of the kind usedhere, their decoders and memories are known in the prior art as apparentfrom the U.S. Pat. No. 3,665,313.

All the portable radio transmitters in an installation are intended tosend on the same radio channel with the same frequency.

The portable alarm senders can also be provided with a memory for apermanently stored code which is unique to a particular alarm sender. Inan emergency, this code is transmitted immediately after the code whichis unique to the location from which the alarm has been sent, thusindicating in the alarm centre which sender and thus which person hassent the alarm.

BRIEF DESCRIPTION OF THE DRAWINGS

An installation in accordance with the invention will now be describedwhile referring to the accompanying drawing in which:

FIG. 1 is a principle block diagram of the installation;

FIG. 2 is a block diagram of an alarm sender and a central alarmreceiver;

FIG. 3 is a block diagram of a locally fixed transmitter; and

FIG. 4 is the floor plan of a building with examples of the placement offixed transmitters.

PREFERRED EMBODIMENT

The installation in accordance with the invention includes, asillustrated in FIG. 1, a plurality of locally fixed transmitters, ofwhich two, 1A, 1B are shown in the Figure. The installation furthercomprises one or more alarm senders 2 and a central alarm receiver 3.

The alarm sender 2 includes, as is apparent from FIG. 2, an antenna 21for receiving inductively transmitted signals; a receiver 22 withamplifier, connected to the antenna 21; a decoder 23 for receivedsignals; a memory 24 for storing a received code, connected to thedecoder; a control logic circuit 25, operable by means of a push button26, for controlling the transmission of the code stored in the memory24, with the help of a radio transmitter 27 and antenna 28, connected tothe logical circuit 25.

As is apparent from FIG. 2, the central alarm receiver 3 includes areceiver antenna 31 for radio signals, a radio receiver 32 withamplifier and a decoder with indicator 33 for displaying receivedmessages.

When an alarm is to be sent by the sender 2, e.g. in a case of assault,the button 26 on the sender carried in the pocket of the wearer ispressed by the wearer. The control logic circuit 25 will thus becomeoperable for transmitting, by means of the radio transmitter 27, radiosignals coded with the room code stored in the memory 24 and unique tothe place where the sender is at the moment.

The alarm sender radio transmitter 27 transmits with a frequencyselected in the 160 MHz-waveband, for example.

The radio signal from the sender is received by the central alarmreceiver 3 and the unique code is shown on the display 33.

In some installations, the sender 2 is also provided with a secondmemory 29. In this memory there is stored a second code unique to theindividual sender, and thus also to the person carrying the sender. Thecontrol logic circuit 25 is adapted for sending the second codeimmediately after the first code when an alarm is sent. The display 33in the central receiver shows the designation of the person who has sentthe alarm.

Such techniques are notoriously wellknown and are shown and describedwith respect to apparatus for transmitting and indicating coded radiosignals in said U.S. Pat. No. 3,678,391.

The room code stored in the memory 24 is transferred inductively to thecarried alarm sender when it comes in the immediate vicinity of one ofseveral fixed transmitters in the installation. In each such fixedtransmitter 1 there is included, as is shown in FIG. 3, a modulatableoscillator 41, a code generator 42 with a code memory connected to theoscillator, an output amplifier 43 connected to the oscillator and amagnetic dipole antenna 44 connected to the output amplifier. A voltagesupply unit 45 is connected to the three first-mentioned part units forsupplying them with the necessary voltages.

The fixed transmitters 1 are placed in the installation, e.g. as isapparent from FIG. 4. The figure is a floor plan of a building withrooms A,B,C and D. So that the fixed transmitters will act reliably onthe carried alarm senders, they are placed in door frames or in othernarrow passages. The transmitting power from the fixed transmitter, e.g.transmitter 51 in FIG. 4 is small, and the field strength from the smalldipole antenna of the transmitter is inversely proportional to the cubeof the distance from the antenna, to a first approximation, it thusbeing ensured that the field strength of the transmitter in a door frameclearly dominates over the field strength of transmitters in other doorframes, e.g. transmitter 52.

If a door is especially wide, two dipole antennae 53A, 53B may beneeded, one in each door jamb and connected to the same fixedtransmitter. A magnetic loop can alternatively be arranged around thedoor frame instead of the dipole antennae.

The inductive transmission enables the fixed transmission to bemanufactured in a single monolitic circuit. The dipole antenna, monolithand voltage supply unit are molded together to one unit for fitting intothe door jamb.

The fixed sender 1,51 transmits a code continuously and inductively,such code being unique to the door etc. where it is placed, and when analarm sender 2 is carried through the doorway the voltage will besufficiently great in its receiver antenna 21 in FIG. 2 for the code tobe written into the sender memory 24. If the sender is subsequentlycarried past another fixed transmitter, e.g. 52, its unique code,differing from the previous one, is transmitted, and the new code iswritten in over the one already in the sender memory. In this way thealarm sender will always be updated with the code unique to itsmomentary location, and it is this code which is transmitted when analarm signal is sent.

Parity bits are added to the typical code so that the right codes willbe received in the alarm sender, in spite of strongly varying fieldstrengths at its receiver. Reception reliability can be furtherincreased by exchanging the latest code put into the sender memory 24only when two new identical and correct codes have been received inimmediate sequence. The transmission rate should be at least five codesper second so that two codes will be reliably received during a hastypassage through the doorway. The fixed transmitters transmit a frequencyselected in the range of 16-140 kHz.

Apparatus for inductively transferring coded signals, their decoders andmemories are known in principle, e.g. from inductive staff locatorequipment, and therefore no detailed description thereof is considerednecessary.

A fixed transmitter, e.g. 54 in FIG. 4, can be placed in a door frame ofa room C, so close to an adjacent room D that in a corner of the room Dan alarm sender, which has been given a unique code set by thetransmitter 55 could get the code altered by the transmitter 54, whichwould be incorrect. Such unwarranted alteration of the typical code isprevented by arranging a dummy transmitter 56 between both fixedtransmitters. The dummy 56 is substantially the same as the other fixedtransmitters, but is adapted for only sending a carrier wave with thesame frequency as the one from the fixed transmitters. The dummytransmission blocks the receiver in the alarm sender and preventsreception of such transmission which can lead to resetting the storedcode. The dummy transmission can also be modulated to obtain greaterblocking action.

The radio transmitters of the alarm senders are normally adapted fortransmitting at the same frequency. The risk of two alarm senderssending an alarm simultaneously and thus blocking each other is verysmall. If it is feared even so that alarms could be sent simultaneously,the risk of blocking can be reduced by making the alarm transmissionshort, e.g. 70 mS and repeating it after a comparatively long intervalof 2 seconds. The risk of blocking is further reduced if the intervalsbetween the alarm transmissions are made somewhat different forindividual alarm senders.

What we claim is:
 1. An alarm system for use in an area having aplurality of localized regions for indicating in which of the localizedregions the alarm is given, said alarm system comprising: a centralalarm receiver means for receiving coded alarm signals and giving areaindications in accordance with received coded location-identifying alarmsignals; a plurality of fixed short range transmitter means, each ofsaid fixed transmitter means being in one of the localized regions forinductively transmitting coded location-identifying alarm signals uniqueto the localized region only within a short range associated with saidregion and not to said central alarm receiver means; and a plurality ofpersonal portable alarm senders, each of said senders including areceiver means for receiving and storing the coded location-identifyingalarm signals emitted by the fixed short range transmitter means when inclose proximity thereto so that as a portable alarm sender is movedthrough the area it stores the coded location-identifying alarm signalsassociated with the localized region of its instant location to theexclusion of other coded location-identifying alarm signals, and a fixeddummy transmitter means in proximity with two of said fixed transmittermeans in an area where the two transmitters having an area of partiallyoverlapping transmission ranges for preventing the receiver means of asender in that area from receiving and storing any codedlocation-identifying alarm signal other than the one already stored, andeach of said senders including user-operable transmitter means foremitting stored coded-location alarm signals to said central alarmreceiver means, said user-operable transmitter means having a rangewhich is greater than the range of said short range transmitter means.2. The system of claim 1 wherein the transmitter means of each senderwhen energized periodically emits the alarm signals, the repetitionperiod being considerably longer than the time for emitting the alarmsignals themselves.
 3. The system of claims 1 and 2 wherein each senderincludes further means for storing coded sender-identifying alarmsignals so that the associated transmitter means emits both codedlocation-identifying alarm signals and coded sender-identifying alarmsignals.